1 Field of the invention
This invention relates to a photoresist process and to photoresist compositions comprising a soluble resin, a photosensitizer and a solvent. Particularly, this invention relates to a photoresist process having advantages including high resolution capability and to improved components of photoresist compositions, such as resins, photosensitive components and solvents.
2. Description of Prior Art
Various attempts have been made in the prior art to improve selected properties of photoresist compositions by using selected novolac resin formulations. For example, in U.S. Pat. Nos. 4,377,631, 4,529,682 and 4,587,196, specific novolac resins made from mixtures containing meta- and para-cresols or ortho-, meta- and para-cresols were described as having utility in positive photoresist compositions with increased photospeed. In U.S. Pat. No. 4,551,409, a naphthol containing resin and mixtures of the naphthol resin with another compatible resin are used in photoresist compositions to increase the heat distortion temperature of the photoresist. These prior-art compositions, while providing improved photospeed and improved thermal properties, do not provide particularly high contrast and resolution. This is a significant drawback in many industrial manufacturing processes for electronic circuit components that require fine-line resolution.
In Materials for Microlithography, L. F. Thompson, G. G. Wilson, and J. M. Frechet; Eds.; ACS Symposium Series 266, American Chemical Society, Washington, D.C., 1984, Chapter 17, p 339., a metacresol-benzaldehyde novolac resin was formulated with a photosensitizer and solvent to produce positive-toned images when the photoresist mixture was applied to a silicon wafer, exposed to actinic radiation and subsequently developed. However, the synthesis of the novolac resin, as taught, produces a material having low molecular weight, and photoresist compositions based on it have low photospeeds and resolution.
Also, it is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473 and 4,409,314 and European Patent Application 0092444. These include alkali-soluble phenolic-formaldehyde novolac resins together with light-sensitive materials. Examples of the light sensitive materials are diazoquinones (DAQs) such as the sulfonate and carboxylate esters and the sulfon- and carbonamides obtained by reacting, respectively, oxo-diazonaphthalene sulfonyl and carbonyl halides with hydroxy, polyhydroxy, amino and polyamino ballast compounds (See U.S. patent application 174,556 filed on Jul. 18, 1950 by Maximillian Paul Schmidt and now abandoned, and U.S. Pat. Nos. 3,046,110, 3,046,122 and 3,046,123). The resins and sensitizers are dissolved in an organic casting solvent or mixture of casting solvents and are applied as a dried thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the admixed naphthoquinone sensitizer acts as a dissolution inhibitor with respect to the resin. Upon exposure of selected areas of a coated substrate to actinic radiation, the sensitizer undergoes a radiation induced chemical transformation, and the exposed areas of the coating are rendered more soluble than the unexposed area. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution, while the unexposed areas are largely unaffected. This produces a positive relief resist pattern on the substrate. In most instances, the imagewise-exposed and developed resist pattern resulting on the substrate will be subjected to treatment by a substrate-etchant process. The photoresist pattern on the substrate protects the resist coated areas of the substrate from the etchant, and thus the etchant is only able to etch the remaining uncoated areas of the substrate which, in the case of a positive photoresist, correspond to the areas previously exposed to actinic: radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create the latent images in the resist prior to development. The relief pattern of photoresist on the substrate produced by the method just described is useful for various applications, including the manufacture of miniaturized
The term PAC as used in this present invention refers to the photoactive component of the resist composition. The PAC generally is sensitive to energetic forms of radiation such as ultraviolet (UV) light, undergoing radiation-induced chemical transformations upon exposure to such radiation.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast and resist resolution capability and resist sidewall angle or wall profile, and resist adhesion. Increased photospeed is important for a photoresist, particularly in applications where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and monochromatic filters. Development contrast is a measure of the photoresist's ability to faithfully transfer the mask dimensions through the entire thickness of the photoresist. Ideally the opening at the top of the photoresist film should have the same dimensions as at the bottom of the film. A resist with enhanced contrast generally has improved edge acuity and enhanced resolution capability.
Resist resolution refers to the capability of a resist system to reproduce with a given phototool the smallest multiple equal line/space features of a mask which is utilized during exposure with a high degree of image edge acuity in the developed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very narrow lines and spaces. The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of very large scale integrated (VLSI) electronic circuits on silicon chips. Circuit density on such a chip can only be increased, assuming lithographic techniques are utilized, by increasing the resolution capabilities of the resist.
Various attempts have been made in the prior art to alter photoresist compositions in order to improve photoresist properties. For example, U.S. Pat. No. 4,688,670 describes a triester-DAQ (tri-DAQ) PAC which achieves a moderate contrast. The composition described contains a PAC and a resin of different chemical structures than those of the present invention, resulting in inferior performance properties.
European Patent Application 85300184.0 describes several photoresist compositions containing polyester DAQ (poly-DAQ) PACs. Many of the PACs claimed in said patent application have only minimal solubility and aging stability in useful photoresist solvents. An overwhelming majority of the PACs in said patent application are esters of highly absorbant hydroxy-functional ballast molecules which have the property of a high absorbance and high non-bleachable absorbance. The absorbance properties of such compositions degrade resolution capabilities. Many of the PACs claimed in said patent application absorb in the visible spectrum, degrading mask alignment techniques. Also, an overwhelming majority of said photoresist compositions in said patent application have the property of poor yield of unexposed (or slightly exposed) film thickness remaining after development: i.e., in the range of 87-93 percent. The compositions taught in said patent application are different from those of the present invention with respect to PAC structure, PAC performance, resin composition and solvent composition. The photoresist compositions of the present invention have markedly better performance with respect to resolution capability, unexposed film thickness remaining after development, absorbance properties and visible transparency.
Japanese Patent 61/45420 A2 describes a photoresist composition containing a poly-DAQ PAC. The PAC, resin and solvent are different from those of the present invention. The performance properties of said composition, especially with respect to unexposed film thickness remaining after development, are inferior to that of this present invention.
Japanese Patent Application 8525660 (850213) describes a photoresist composition containing a PAC with a poly-DAQ PAC. The PAC, resin and solvent are quite different from those of the present invention.
Japanese Patent Application 84239330 (841115) describes a photoresist composition containing PACs, some of which contain a poly-DAQ PAC. The PAC, resin and solvent are of quite different composition from those of the present invention. In addition, the degree of PAC esterification claimed is less than the range of the present invention.
U.S. Pat. No. 4,555,469 describes a photoresist composition containing PACs, some of which may contain a poly-DAQ PAC. These PACs are esters of a novolac resin and are of a polymeric nature with structures different from those of the present invention. There are at most very minimal amounts of fully esterified PAC in the compositions of said patent, because the fully esterified novolac resin is minimally soluble in useful photoresist solvents. As shown in the present invention, the lack of fully esterified PACs lessens the resolution capability of photoresists based on such PACs. Also, the resin structure and solvent composition of said patent are different from those of the present invention.
The art of conventional positive photoresists is well known. For example, U.S. Pat. Nos. 4,550,069 and 4,529,682 describe the principles of positive photoresist compositions, including differential solubility, contrast, photospeed, resolution, unexposed film thickness loss and applications. These and other patents describe typical embodiments, including particular plasticising resins, dyes, anti-striation agents, adhesion promoters, and speed promoters.
With respect to the solvent component, an acceptable solvent must be capable of dissolving the required amounts of photosensitizer or PAC. Typically a photoresist formulation must contain between 2.5 and 5 percent by weight PAC in order to render the dried unexposed photoresist film sufficiently insoluble in an aqueous alkaline developing solution. A widely used casting solvent composition for positive photoresists consists of a mixture of ethylene glycol monoethyl ether acetate, n-butyl acetate, and xylene in the ratio of 80:10:10, as taught in U.S. Pat. No. 4,550,069.
The difficulty in dissolving PACs having multiple diazoquinone moieties with traditional solvents is a well known aspect of the prior art, as shown in European Patent 0,126,266 A2. Accordingly, in view of solubility limitations, it is common prior-art practice to use a mixture of sensitizers consisting of less soluble and more soluble types, or even to operate with the solution supersaturated relative to the less soluble sensitizer component. This practice can lead to precipitation of the sensitizer prior to or during use of the photoresist composition, and consequently can result in a short shelf life. There are several approaches to overcoming the solubility limitations. Among them are: (1) using a more soluble photosensitizer, (2) using a more effective solvent, and (3) using filtration of the composition immediately at the point of use. U.S. Pat. No. 4,266,001 discloses certain organic esters of diazonaphthoquinone sulfonyl chloride (i.e. oxodiazonaphthalene sulfonyl chloride) that are distinguished as having a high solubility in traditional photoresist solvents. Some of the prior-art photosensitizers consisting of certain esters of 1-oxo-2-diazonaphthalene-5-sulfonic acid are among the more soluble of PACs in traditional solvents, while many variations of these PACs, such as certain esters 1-oxo-2-diazonaphthalene-4-sulfonic acid are known to be less soluble, as discussed in B. Z. Yakovlev et al., Khim. Prom-st., 9, 564, 1981.
Traditional solvents used in the prior art include: ethyl cellosolve or ethylene glycol monoethyl ether, ethyl cellosolve acetate or ethylene glycol monoethyl ether acetate, methyl cellosolve or ethylene glycol monomethyl ether, methyl cellosolve acetate or ethylene glycol monomethyl ether acetate, N,N-dimethyl formamide, dioxane, and cyclohexanone, as shown in Japanese Patents 8286548 and 81202455. In order to improve coatability, up to 20 percent by weight of other solvents such as xylene, n-butyl acetate, and cyclohexane are included. Propylene glycol monoalkyl ethers are perceived by some people to be biologically safer solvents than the traditional ethylene glycol analogs (European Patent Application 85106774). Propylene glycol monoalkyl ether acetates are attributed to impart a photoresist photospeed advantage (U.S. Pat. No. 4,550,069). Also, solvent combinations, such as solvents with 60.degree. to 170.degree. C. boiling points admixed with solvents with 180.degree. to 350.degree. C. boiling points, are claimed to eliminate coating striations as described in Japanese Patent Application J60024545-A.
Other solvents or mixtures of solvents have been discussed in the art as providing photoresist compositions of exceptional stability to storage. The active solvent in many of these formulations consists of pure, or in part, cyclopentanone and cyclohexanone as shown in Japanese Patent Application 84/155838. Combinations of cyclopentanone and cyclohexanone with 5- to 12-carbon aliphatic alcohols also are attributed to have good coating performance, as described in European Patent Application 0,126,266. Mixtures of acetone, ethylene glycol, and water are reported to form highly stable photoresist solutions, as shown in German Offen. DE 2319159.
Nearly all of the solvents described above, as well as others in the prior art relating to photoresist compositions containing diazonaphthoquinonesulfonate or carboxylic acid esters of trihydroxybenzophenone, are limited in their applications. This is particularly the case when contemplated for industrial use where the combination of good solution stability, low toxicity, acceptable margin of safety against fires and explosions, and good coating characteristics is important. For example, methyl cellosolve acetate, N,N-dimethylformamide, and cyclohexanone have been implicated as biological threats to worker safety. In addition, structurally related alkylene glycol monoalkyl ethers and their acetates are potential biological threats to worker safety, as reported in the NIOSH Current Intelligence Bulletin 39, May 391983.
In order to provide an optimal margin of safety against fires and explosions, it is desirable for the photoresist composition to have as high a flash point as possible. This substantially restricts the use of highly volatile solvents such as acetone, cyclopentanone, and 1,4-dioxane, since these solvents have unacceptably low flash points. Since flash point generally correlates well with boiling point, the need for safety against fire and explosions places substantial restrictions on using solvents with boiling points below 110.degree. C. in photoresist compositions.
In addition to safety issues, the use of highly volatile solvents presents severe coating problems as well. Since efficiently practicing the art of high resolution microlithography requires maintaining critical dimension control from wafer edge to wafer edge, it is essential for a photoresist coating to have an extremely high degree of thickness uniformity, typically less than 50 angstroms variation across a 100 mm diameter substrate. Solvent mixtures containing as little as 5 percent of solvents with boiling points below 110.degree. C. tend to produce unacceptable thickness variation, because the drying time is likely to be too short compared to the spreading time in a typical spin coating process.
On the other hand, it is important for a photoresist solvent to be volatile enough to evaporate nearly completely during the coating process. This requirement places severe restrictions on the use of relatively high boiling solvents in photoresist compositions, especially since the post spin bake temperature cannot be raised much above 110.degree. C. without causing substantial thermal decomposition of the photosensitizer.
Finally, the distinction between physical solution stability and chemical reactivity should be made. As discussed above, the literature has a number of examples of highly stable photoresists based on solvent mixtures containing cycloaliphatic ketones. Although these photoresists are highly stable in the sense that the photosensitizer does not precipitate out of solution, they are chemically unstable. The diazo moieties are reactive towards cycloaliphatic ketones. It is well known that the color of solutions of photosensitizer and cyclopentanone, for example, turns from red to black on standing at room temperature for just a few days. Opening sealed sample vials of such solutions is accompanied by audiable release of pressurized nitrogen gas, indicating that the photosensitizer has undergone decomposition. The same is true of many amines, even to a higher degree. It is essential for industrially viable photoresist compositions to be free of such degradation effects.